Gas phase filtration has traditionally been accomplished by low, medium and high efficiency pleatable composite filter media which include either a low, medium or high efficiency fibrous filtration layer of randomly oriented fibers; and one or more permeable stiffening layers which enable the composite filter media to be pleated and to sustain its shape. Such filtration devices serve as vehicle passenger compartment air filters, high performance engine air filters and engine oil filters. ASHRAE (American Society of Heating Refrigeration and Air Conditioning Engineers) pleatable filters and the like typically use a pleated high efficiency filtration media for the filtration element.
Pleatable composite filtration media made of a nanofiber high efficiency layer and a more permeable spunbound fiber stiffening layer (also referred to as an “SN” media) have been shown to give good flux/barrier properties (i.e. high efficiency and low pressure drop). However, the dust-loading capacity is lower than the desired value in certain industrial HVAC applications when filters are challenged with very small dust particles, which can occur when the HVAC system is designed and constructed to have lower efficiency pre-filters in front of the high-efficiency final filters. In the SN structure, the scrim is typically made of nonwoven webs of fiber diameter of 14 to 30 microns which can pre-filter out particles larger than about 5 microns in size. The remaining particles will reach the thin nanofiber layer and quickly fill up the pores and plug up the filters. As the result, filter resistance increases rapidly and thus shortens filter life. Attempts have been made to increase the dust-loading capacity by increasing the basis weight and thickness of the scrim layer but the results are still unsatisfactory for the more demanding situations.
To further complicate the problem, when the humidity of the incoming air is high or the incoming air contains a water mist, dust loaded on the nanofiber layer of the filter media can pick up moisture and swell. It is widely known that a high percentage of atmospheric aerosol is hydroscopic in nature. This further reduces the remaining pore size and creates additional flow restriction and increased pressure drop across the filters. These spikes in pressure drop can create significant problems to HVAC systems.
There remains a need to provide a relatively low cost, high efficiency filter media for these filtration applications which exhibit relatively high dirt-holding and/or air contaminant capacities and relatively low pressure drops in the presence of moisture. One object of the present invention is to provide such a filter medium and a method for use of the same.